The performance of the free electron laser for the directed energy weapons application depends critically on only three technologies: the high brightness accelerator, the FEL amplifier, and the cavity optics. Our goal in this program is to examine the performance of the optical elements under the combined loading of the fundamental and the harmonic emissions of the FEL amplifier. The performance of both the RF linac based FEL and the induction linac based FEL may be subject to significant limitations due to UV-induced absorption in the early optical elements following the FEL amplifier. These limitations are expected to appear suddenly at a certain fluence level, with little warning. Our central objective in this program is to measure the rate of the UV induced absorption, and to identify the scaling parameters so that we can predict with some confidence the performance limitations of the optics. When these limits are reached in the experimental programs, a knowledge of the scaling relations and possible countermeasures will be invaluable in planning an appropriate response to the problem.

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